Various systems use transponders to identify objects from a distance. Transponders attach to or are otherwise associated with the objects to be identified. The transponders are programmed with unique identification codes. Typically, an interrogator transmits an interrogation signal. When a transponder receives the interrogation signal, it responds by broadcasting its unique identification code. The interrogator identifies the transponder and the object with which it is associated by detecting this identification code.
Numerous diverse applications currently use such identifying transponders. However, the high cost, excessive bulk and poor reliability of conventional transponders prevent their use in numerous other applications. A highly reliable, small transponder which may be provided at such a low cost that it may be regarded as a simple "tag" is needed to permit these and many other applications to identify objects from a distance. These applications may include, for example, tags for identifying luggage, employees, vehicles, goods being manufactured, goods in inventory and many others.
As an example, conventional small and inexpensive transponders use a circuit board about the size of a credit card. In order to minimize bulk, reduce complexity, and improve reliability, such transponders may be configured as passive transponders. In other words, such transponders may not include their own source of electrical energy, such as a battery. Rather, they may obtain the energy required for their operation from the interrogation signal itself. The response signal may be broadcast by modulating the reflectivity of the transponder to the interrogation signal rather than by actively sourcing and radiating an RF signal.
Such conventional transponders use an antenna printed on one portion of the circuit board and mount various discrete, individually packaged electrical components, such as semiconductor chips, diodes, resistors, capacitors, inductors and the like on remaining portions of the circuit board. The antenna size is limited due to the need to allocate circuit board area to the electrical components. This limited antenna size likewise limits the transponder's ability to collect electrical energy from the interrogation signal. A slow or weak response signal results.
Moreover, the individually packaged, discrete electrical components mounted on the circuit board cause the transponder to be undesirably thick. The individually packaged components typically must be protected from the elements by placing the transponder within yet another package or housing. This overall transponder package or housing further increases the transponder's thickness. This excessive thickness prevents, for example, the transponder from being carried in a wallet. Likewise, this thickness prevents the transponder from being used in many applications because the transponder would impose an obstruction that would physically interfere with movement or normal jostling experienced by an object to which it might be attached.
Furthermore, the individually packaged, discrete electrical components mounted on the circuit board cause the transponder to be undesirably complex and unreliable. Complexity increases, due to the individual packaging material and processing, the additional overall transponder housing and the increased handling and testing associated with the individually packaged components. Reliability decreases due the minimal shielding provided for such components, leading to interference and other noise-related problems caused by strong interrogation signals and other electrical fields. The interference may cause improper response signals and even corrupted programming for identification codes stored within the transponder.